The present invention relates to an improved method of operating a magnetohydrodynamic power plant and, more specifically, to an improved method of operating a MHD power plant whereby the flue gas released to the atmosphere has a low content of nitrogen oxides.
The fundamental principles of generating electricity via a magnetohydrodynamic (MHD) power plant are well-known in the art. Basically, a high temperature, pressurized gas is produced in a combustor by burning a fossil fuel such as coal in air or oxygen. A seed material which is readily ionizable at the gas temperatures within the combustor is introduced into the gas to form the working fluid for the MHD generator. This working fluid is then passed through the MHD channel of the MHD generator wherein the working fluid passes at a high velocity through a magnetic field to generate electricity. The working fluid then exits the MHD generator through a diffuser section to various heat transfer sections and dust collection equipment disposed downstream of the MHD generator for cooling and cleaning the gas before it is vented to the atmosphere.
One of the major problems associated with generating electricity by magnetohydrodynamics is air pollution. In a typical prior art MHD power plant, coal is burned in air and under pressure to produce a gas having a temperature of approximately 2500 C. or more. At these temperatures, the formation of very high levels of oxides of nitrogen during the combustion process within the combustor is unavoidable. Nitrogen oxide concentrations in the gases leaving the combustor in the vicinity of 10,000 ppm are common.
Naturally, it is environmentally prohibitive to release such a concentration of nitroge oxides to the atmosphere. Therefore, the gas produced in the MHD combustor must be processed to reduce the nitrogen oxide concentration therein before the gases are vented to the atmosphere.
The most common method of reducing the nitrogen oxide content in the gases formed in the MHD combustor comprises placing a dwell chamber immediately downstream of the MHD generator. The gases leaving the diffuser section of the MHD generator are passed to the dwell chamber and held therein for a period of time of typically at least 2 seconds at a temperature above 1600 C. in order to allow the oxides of nitrogen in the gas to naturally decompose to an equilibrium concentration at the gas temperature within the dwell chamber. In order to ensure that the proper temperatures are maintained and sufficient residence time is provided for the nitrogen oxides to decompose, a typical dwell chamber is a fairly massive refractory-lined furnace structure which is expensive to build and takes up much space.
To further assist in reducing nitrogen oxide concentrations in the gas produced in the combustor, it is common in the prior art to burn the coal in the combustor at slightly substoichiometric conditions. In this manner, a slightly fuel-rich product gas is generated containing carbon monoxide which acts as a reducing agent to attenuate nitrogen oxides formation. A strong reducing atmosphere in effect accelerates the decomposition of the nitrogen oxides. Nevertheless, dwell chambers are still required even though the combustor is operated slightly substoichiometrically. Although the concentration of nitrogen oxides in the gas produced in the combustor is less when the fuel is burned with substoichiometric air, the residence time and the bgas temperature required in the dwell chamber to permit the oxides of nitrogen to decompose to their equilibrium leves is still prohibitively high.
In addition, when the fuel is burned in substoichiometric air in the combustor, a secondary furnace must be provided downstream of the dwell chamber to combust ay uncombusted fuel remaining in the bases before the gas can be vented to the atmosphere.